The present invention relates to a plunger suitable for use in a glass forming machine, for example, of the I.S. type, in forming gobs of molten glass into parisons for subsequent formation into hollow glass articles. The plunger is actuated by an electro servo-driven leadscrew which works in conjunction with a position-sensing resolver to provide plunger position information. Although the instant application primarily addresses the "narrow neck press and blow" ("NNPB") process of glass making, applications as to other types of processes, such as "press and blow,"("P&B") or "blow and blow" ("B&B") will become evident as further described.
I.S. types of glass forming machines are well known in the industry. These machines have a number of individual forming units, or sections, each of which receives gobs of molten glass from a common source and feeds their output to a common conveyor. For the NNPB process, each section has at least one parison forming mold in which gobs of molten glass are formed into parisons and at least one blow mold in which the parisons are blown to the required shape.
In the NNPB process, parisons are formed when a molten gob of glass is delivered to a mold cavity of a press mold and then pressed by a plunger mechanism against the walls of the mold cavity, simultaneously forming the inner surface of the parison (dictated by the shape of the plunger head) and outer surface of the parison (dictated by the shape of the mold cavity). A conventional plunger is usually made up of a cylinder located below the press mold and a piston in the cylinder which is movable towards and away from the mold cavity upon the introduction of fluid under pressure into the cylinder. A piston rod projects from the piston towards the mold cavity and is arranged to carry the plunger so that the movement of the piston causes the movement of the plunger towards and away from the mold cavity.
In a conventional NNPB process, the piston typically uses air pressure to move the plunger into the mold cavity so that a plunger head affixed to the plunger presses the gob of molten glass into the shape of the mold cavity. After the pressing of the gob is complete, the same air under pressure is then used to move the plunger fully out of the mold cavity to the "plunger down" position so the parison can be moved to the next station. After the parison is moved to the next station to be blown into the hollow glass article, the plunger is moved to an intermediate "load" position while the next gob of glass is delivered to the mold cavity, and the process is repeated.
The disadvantages of such a system revolve around factors such as precise control of the actuating air pressure, properties of the liquid glass (which are constantly changing), and the quality of parisons produced due to the individual characteristics of each plunger. For example, the glass pressing pressure in a conventional pneumatic system is typically controlled by a pressure regulator. One pound of change in controlling air pressure results in a twelve pound change in pressing pressure. Consequently, any error in control air pressure is also amplified by a factor of twelve.
Furthermore, variables related to the movement of the plunger, such as the static pressure on the gob of glass and the speed of the plunger, typically depend upon the time axis. That is, in a conventional process, the plunger maintains a certain position for a calculated length of time and then moves to the next position, regardless of system dynamics. This requires precise synchronization of gob delivery, plunger movement, and parison transport, which is difficult at best, and provides no feedback for system correction during operation.
Previous inventions, such as that disclosed by U.S. Pat. No. 5,236,485, attempt to eliminate such problems by utilizing the position of the plunger in lieu of the pneumatically controlled movement of the plunger dependent upon the time variable. In that patent, Leweringhaus teaches a plunger actuated by an "electro-hydraulic power drive." A piston and cylinder arrangement is used as the plunger movement is controlled by hydraulic fluid. A valve is controlled electrically to increase or decrease prescribed amounts of hydraulic fluid from both ends of the piston, allowing for all intermediate positions for the advancing and retracting of the plunger to be available. The actual position of the plunger relative to the cylinder is monitored and compared to predetermined stored values, i.e., the desired positions, thus allowing the feedback to dictate the function of the hydraulic control valve and ultimately the position of the plunger. This eliminates the time variable in the equation, allowing for a more accurate and efficient glass making process.
While the use of a hydraulically-actuated plunger addresses the time dependency problem inherent in previous technology, any system dependent upon the position of the plunger is critically dependent upon the accuracy of the position detection components of the device, which may not always be the most precise. The Leweringhaus invention still relies upon traditional position detection and indication of the plunger, i.e., a coil-and-core type sensor. One such coil-and-core type sensor is disclosed in U.S. Pat. No. 4,613,352 issued to Mannfred Krumme.
In the Krumme patent, a ring-shaped core is carried by a piston rod mounted to the plunger of the glass-forming machine. The core forms an actuating element for changing the inductivity of a coil which is arranged in a ring-shaped frame between a cylinder and a guiding cylinder for the plunger. During each working stroke of the plunger, the maximum insertion depth in the mold is measured and used to generate an analog electrical signal. The signal is then compared to a reference value which, in turn, provides an adjusting value for adjusting the mass of the gob prior to delivery. One disadvantage, however, of the core-and-coil type sensor is that the linear position of the plunger cannot be measured for the full stroke of the plunger. The disclosures of the aforesaid U.S. Pat. Nos. 5,236,485 and 4,613,352 are incorporated by reference herein.